In the production of high quality fine filament NbTi superconducting wire, manufacturers must rely heavily on the integrity of diffusion barriers. These barriers, usually Nb, are positioned between the copper cladding and the NbTi superconducting alloy that constitutes the bulk of a filament. The barrier serves to inhibit the formation of brittle CuTi intermetallics at the filament surface. Current processing schemes for the production of NbTi superconducting wire containing 4000-7000 filaments, each about 6 microns in diameter at the final wire size, utilize Nb barriers that constitute about 4% of the filament (non-copper) volume. The processing will typically subject the barrier to 500.degree.-800.degree. C. temperatures over a period of approximately 15 hours, followed by three or more 300.degree.-450.degree. C., 40-80 hour heat treatments as the wire is brought to final size. Current densities (J.sub.c 's) in excess of 2750 A/mm.sup.2 at 5 Tesla (T) and 1600 A/mm.sup.2 at 7 T (4.2.degree.) can be achieved in fine filament conductors processed in this way (see, for example, "Superconducting Wire and Cable for the Superconducting Super Collider", T. S. Kreilick, E. Gregory, D. Christopherson, G. P. Swenson, and J. Wong, Supercollider 1, Plenum Press, 1989, 235-243).
While not unacceptable, these current densities are well below the 3800 A/mm.sup.2 (5 T, 4.2.degree.) that has been achieved in wires with larger diameter filaments. The degradation in current density as the filaments grow finer is directly attributable to problems with the diffusion barrier.
The long periods at elevated temperatures during processing tend to undermine the Nb barrier due to Ti diffusion from the NbTi alloy core. It must be remembered that as Ti diffuses into the pure Nb barrier, Ti is depleted from the surface of the NbTi alloy filament core, resulting in lower overall J.sub.c in the core. The diffusion of Ti through the barrier also results in CuTi compound formation at the surface of the filament. This brittle CuTi compound fractures during cold reduction, resulting in "nodules" that adversely affect both the J.sub.c and the ductility of the wire. The surface interfaces between the copper matrix, the Nb barrier, and the NbTi core in a filament play a central role in this process.
In cases where irregular interfaces occur, extensive localized thinning of the diffusion barrier is observed. The effect is most pronounced in areas where there are projections into the barrier and even into the surrounding copper. It is believed that these areas are where CuTi compound will first form, and in greater quantity than in areas that do not display such thinning. These CuTi regions, the nodules, fracture during wire drawing and subsequently cause sausaging of the filaments. As a result, the wire displays severely degraded J.sub.c performance and is prone to breakage.
The obvious solution to the problem of diffusion barrier thinning is to simply use a thicker barrier, so that there are no thin spots to cause problems. Unfortunately, an increase in the barrier thickness comes only at the expense of the NbTi. The barrier supports no J.sub.c at operating magnetic fields, so thickening the barrier reduces the overall J.sub.c. In addition, increasing the barrier thickness in no way remedies the problem of Ti depletion in the NbTi. While not so dramatic as the problem of CuTi compound formation, this problem cannot be ignored if current capacity is to be improved. So, while increased barrier thickness is an obvious solution to the problem of barrier thinning, it is far from ideal.
Rather than compensate for surface irregularities by thickening the barrier, we have found that it is better to eliminate those irregularities--i.e., to provide as uniform a surface behind the diffusion barrier as possible. When it is surrounded by a uniform surface, the diffusion barrier becomes more efficient. Nodule formation can be largely eliminated and, to the extent that any barrier might have to be thickened, the necessary increase can be minimized. The production of uniform NbTi surfaces in filaments is the primary objective of the invention.
Whenever two or more metals are co-processed, the degree of irregularity at their interface depends upon a great many factors. Among these are:
1. The relative hardness of the materials. PA1 2. The grain size within the materials. PA1 3. Texture and overall structure within the materials as dictated by prior metallurgical and mechanical processing (hot work, cold work, and total deformation, e.g.).
The ideal interface, from the standpoint of uniformity, is that between two metals of nearly equal hardness, both having very fine grains aligned along the plane of the interface so as to present the smoothest possible surfaces. Clearly, rolled sheet materials fit this description well. The use of rolled sheet for the purpose of increased filament uniformity forms the basis of the present invention. An earlier U.S. Pat. No. 4,646,197, issued to Supercon, Inc., related to the fabrication of Ta capacitor wire, wherein fine grained Ta sheet was wrapped around a Nb or Ta ingot to provide a smooth wire surface with marked resistance to Ta grain growth across the wire at elevated temperatures.